Add a Locale Implementation Guide

## Issue Description

As we've added new features such as dehumanize, it's starting to become clear there are some major discrepancies between locale implementations. I think it would be a good idea to create some kind of adding new locale guide. This would hopefully help avoid some of the issues/ambiguity we've been having (i.e locale implementations using the translation of "week ago" instead of "a week ago" seen in #982).

Review the existing locale implementations and the dehumanize-related behavior, then compare the discrepancy described in #982. The work is complete when a documented guide explains the conventions and steps for adding a locale, including how to avoid ambiguity in translations such as “week ago” versus “a week ago”.

Written by the indexing model from the issue text.